Acid-activated bentonite clays in the form of finely ground powders, commonly called bleaching clays or bleaching earths, are used to adsorb colored pigments, such as carotenoids and chlorophyll, as well as colorless contaminants, such as phospholipids, and hydroperoxides from edible and inedible oils. This process is called "bleaching" and serves both cosmetic and chemical purposes: (i) it reduces color--very clear, almost water white oils are produced that meet with U.S. consumer expectations, and (ii) it stabilizes the oil--both the colored pigments and colorless contaminants tend to destabilize the oil (i.e., cause the oil to become rancid more easily) if they are not removed.
Usually, the bleaching process includes adding a bleaching clay powder to a contaminated oil at levels in the range of about 0.35 to about 3.0 wt %, depending on the type and quality of oil being processed. The clay/oil mixture then is heated to a temperature in the range of about 90.degree. C. to about 120.degree. C. for about 20 to 30 minutes, after which time the now "spent" clay is separated from the bleached oil by filtration, using filter presses or pressure leaf filters. Typically, the filter cake resulting from the filtration operation contains the pigments and other contaminants removed from the oil, as well as considerable quantities, for example, about 25 to about 40 wt %, of entrained oil that is difficult to remove from the spent clay.
It is known that when extra processing, such as solvent or lye-extraction, is employed to remove the entrained oil from the spent clay, the extracted oil is of poor quality. In addition, the extra processing involved is both difficult and expensive. Accordingly, most oil refiners choose to dispose of spent bleaching clay from their operations by having it hauled away to landfill sites.
Unfortunately, spent bleaching clays also possess rather high surface areas, for example, about 250 to about 350 m.sup.2 /gm, and when the spent clays are saturated with entrained oils, they can spontaneously ignite. Because of this pyrophoric tendency, oil-laden spent bleaching clays are classified as hazardous landfill materials and premium rates are levied to dispose of them in landfill sites. From the standpoint of the individual refiner whose margins in this highly competitive industry are tight, these extra expenses are an additional and unwanted burden. From the standpoint of municipalities faced with the problems of acquiring and managing landfill sites, they are becoming increasingly stringent regarding the types of materials they will accept for these sites and striving to encourage recycling as a way of reducing demand. However, even though there has been a long felt need in the industry to recycle the oils and the bleaching clays, until the present invention, there has been no solution to this costly problem.
Although the dosage levels of bleaching clays used to process edible and inedible oils are actually quite low, the magnitude of this problem can be appreciated by considering the fact that about 10 million metric tons of edible and inedible oils were processed in the United States in 1990. At an average dosage level of about 0.75 percent by weight bleaching clay, processing would yield about 75,000 metric tons per year of spent clay plus an additional 30,000 metric tons per year of entrained oils that must be disposed of.
One approach to solving the disposal problems associated with the creation of spent bleaching clays from edible and inedible oil refining processes is to regenerate and recycle the spent clay. Although a number of processes have been described for the purpose of accomplishing this objective, all previous processes suffer from the inability to completely regenerate the spent clay. It is important to appreciate that since bleaching clays are used only once in a batch-type operation, if anything less than complete regeneration is accomplished, the economics of performing the regeneration, coupled with the cumulative loss in bleaching activity, would combine, after only a few cycles, to produce a regenerated clay that would be an unattractive alternative to the use of fresh bleaching clay. Table I shown below can be used to illustrate the loss in cumulative regenerated activity at different (constant) regeneration levels.
TABLE I ______________________________________ % % Regenerated activity remaining Regeneration after cycle per cycle 1 2 3 4 5 6 ______________________________________ 97.5 97.5 95.1 92.7 90.4 88.1 85.9 95 95 90.3 85.7 81.5 77.4 63.5 92.5 92.5 85.6 79.1 73.2 67.7 62.6 90 90 81.0 72.9 65.6 59.0 53.1 ______________________________________
The use of organic solvents presents a number of environmental and safety problems associated with their use. Included among them are the necessity for having: (1) processing systems which have fire and explosion safeguards; (2) equipment for capturing any vapors that might otherwise escape to the atmosphere; and (3) equipment for separating, purifying and recycling the organic solvents. These requirements all add considerably to the expense of utilizing any regeneration process which might require the use of organic solvents.
The possibility for regenerating spent bleaching earths has been studied by other workers. Bahl and Dayal, Research and Industry, 22, 145 (1977), pointed out that the simplest possible method of regenerating bleaching clays after their use in oil refining operations was to subject the oil-soaked clays to a controlled burning operation. They noted, however, that localized overheating of such oil-soaked clays during the burning operation was difficult to control, and further, that no value was derived from the entrained oil. Further, it was reported that with each burning cycle, the activity of the clay declines, and that a portion is lost as fines. Also it was noted that regeneration of finely powdered clays, such as the bleaching powders processed in accordance with the present invention, is rarely economical since the fine powdered clay escapes with the combustion gases during burning.
Simpson, et al. U.S. Pat. Nos. 2,246,127 and 2,449,016 disclose a regeneration procedure for granulated bleaching clays, used to treat petroleum oils, in which at least some residual matter is carbonized and the clay, together with a coating of activated carbon, is a more active adsorbent than the original clay. The Simpson, et al. U.S. Pat. Nos. '127 and '016 patents teach that solvents, used to extract oil from a bleaching clay, are generally not favored commercially because they are too expensive. A number of patents have issued based on organic solvent regeneration, such as disclosed in U.S. Pat. Nos. 2,286,815; 2,370,713; and 3,472,786. Fuege and Janssen, J. Amer. Oil Chemists' Soc., 28, 429 (1951), claim 96-97% regeneration of spent bleaching clays when using polar organic solvents, such as alcohols, ethers, and ketones. However, the degree of regeneration drops to the 79-80% range after about 20 cycles.
Some attempts to regenerate spent bleaching clays use both water and organic solvents during the regeneration process. Zerbe U.S. Pat. No. 2,352,064 discloses treating the clay with benzene and a benzene-alcohol mixture for regenerating spent clay; and then a water treatment. Likewise, Rhenania French Patent No. 869,534 discloses the use of water at the end of the regeneration procedure, after first washing the clay with an organic solvent, such as benzene or toluene, to remove oil and then using a mixed solvent such as benzene and 10% ethanol for extraction of resinous products. Martin U.S. Pat. No. 2,328,158 discloses the use of a blend of about 3 parts naphtha and 1 part of a ketone, and water maintained in a proportion of at least 5% to regenerate spent bleaching clays. Other processes disclose the use of organic solvent extractions in conjunction with high temperature heat treatments for spent clay regeneration, as disclosed in British Patent No. 491,338; Swiss Patent No. 237,380; Indian Patent No. 61,157; and German Patent No. 1,941,758.
More recently, Kalam and Joshi J. Amer. Oil Chemists' Soc., 65, 1917 (1988), report utilizing a hexane extraction step followed by high temperature aqueous regeneration, and point out that although numerous processes for regenerating spent bleaching earths already exist, they are either too complicated or too costly. Because of this, the general practice is still to simply discard the spent, oil-saturated material in landfill sites. In the Kalam and Joshi process, the spent, oil-saturated clay is first extracted with hexane and then dried at 110.degree. C. for six hours to remove all traces of solvent. Afterwards, the clay is autoclaved at temperatures in excess of 235.degree. C. for six hours. Regenerated clay from this process was found to possess about 80% as much bleaching activity as fresh bleaching clay. In a more successful variation of this process Ibid., 65, 1537 (1988), the clay is first extracted with hexane, as before, but then a small partial pressure of oxygen is maintained in the autoclave during the aqueous thermal regeneration. Best results were obtained at low slurry concentrations, e.g., 5 wt % spent clay, when temperatures were in excess of 200.degree. C., with 0.5 MPa of oxygen partial pressure, and conditions maintained for six hours. 100% regenerations were obtained, and could be maintained at that level through at least four cycles.
Waldmann and Eggers, Ibid., 68, 922 (1991), disclose the use of CO.sub.2 under supercritical conditions to extract and regenerate spent bleaching clay. Although good removal of oil was achieved (up to 97% in the case of palm oil), carotenoids present in vegetable oils appear not to be removed by this extraction process. The regenerated clay exhibited excellent (100%) efficiency for removing chlorophyll, but only about 50% as much activity as fresh bleaching clay for removing carotenoids, presumably because carotenoids already present were blocking adsorption sites normally used in the removal of the carotenoids. Model U.S. Pat. No. 4,124,528 discloses a similar process to regenerate spent activated carbon which had been used to treat waste water.
Surprisingly, the process of the present invention for regeneration of bleaching clays eliminates the disadvantages of prior art recycling processes and provides essentially complete regeneration of spent bleaching clays, and acid-activated clay catalysts. The process of the present invention yields a low grade oil from the bleaching clay regeneration process that is suitable for use as a fuel oil, for blending in animal feed, or as a source oil for the production of fatty acids, among other uses.
In accordance with another important aspect of the process of the present invention, granulated acid-activated smectite clay catalysts, e.g., olefin polymerization catalysts and other catalysts, e.g., for alkylation of olefins, in the form of acid-activated smectite clay granules, can be regenerated using the process of the present invention. As disclosed in this Assignee's copending application entitled "PROCESS OF ACID BINDING FINE SMECTITE CLAY PARTICLES INTO GRANULES", Ser. No. 08/011,273, filed Jan. 29, 1993 acid-activated smectite clay-based catalyst, in granule form, is manufactured by acid binding smaller (fine) smectite particles such that the bound clay particles, in the form of acid-bound granules, have sufficient breaking strength and structural integrity to perform as a olefin polymerization catalyst, with substantially increased yield (.apprxeq.100%) due to the capability of recycling all fine clay particles produced in grinding. The bentonite clay-based polymerization catalyst is manufactured by adhering together a plurality of smaller acid-activated bentonite clay particles, using a strong mineral acid, such as H.sub.2 SO.sub.4, as a binder. An acid-activated bentonite clay having a particle size distribution in the range of about 2 microns to about 200 microns, predominantly (&gt;50%) in the range of about 45 microns to about 130 microns, preferably calcium bentonite, having calcium as a predominant exchangeable action, is mixed with a strong mineral acid, preferably H.sub.2 SO.sub.4, in an amount of about 1% to about 5% by weight, and water, preferably in an amount of about 50% to about 65% by weight to form the catalyst. The mixture is subjected to intensive mixing, curing and drying to form strong particles capable of grinding to a desired particle size distribution, e.g., -6 mesh to +60 mesh, U.S. Sieve Series, to form the catalyst. Fines resulting from the grinding step are completely recycled to the intensive mixing stage of the process to achieve 100% yield. Some adjustment of acid content of the mix may be needed to take into account the acid carried into the mix by recycled fines.
The acid-activated smectite clay-based catalyst granules (e.g., formed from acid-activated calcium bentonite), are primarily useful for treating extraction unit extract streams for recovery of aromatic hydrocarbons free from reactive olefins. The clay-based catalyst is characterized by its mildly acidic catalytic activity and at temperatures of between about 150.degree. C. and about 200.degree. C. (about 300.degree. F. to about 400.degree. F.), the acid sites on the clay promote polymerization and alkylation of olefins. Such acid-activated smectite clay-based granules also can be regenerated in accordance with the process of the present invention.